In general, a step actuator for applying the present invention includes: a stator having a bobbin for winding coil, and a yoke; a rotator having a cylindrical magnet disposed inside of the stator with a prescribed gap, and a hollow nut member with a screw thread of a prescribed length formed on inner circumference thereof, for being inserted and secured in the cylindrical magnet, whereby a lead screw coupled with the screw thread formed in the nut member moves back and forth in an axial direction responding to a rotation of the rotator to transmit power in the axial direction.
Such a step actuator conventionally includes a cylindrical bearing at the upper part of the nut member, for supporting the rotation of the nut member and maintaining the rotator and the stator coaxially surrounding the rotator. The cylindrical bearing is generally coupled to the upper part of a hollow shaft of the rotator. To prevent axial movement of the cylindrical bearing and the rotator, a stop ring engaging recess is formed to receive a stop ring at an area of the hollow shaft of the rotator projected over the upper surface of the cylindrical bearing, and a bearing support for supporting the cylindrical bearing is provided at an area of the hollow shaft of the rotator confronting a lower surface of the cylindrical bearing, thereby preventing the cylindrical bearing and the rotator from moving in an axial direction. In this structure, the distance allowance between the bearing support and the stop ring engaging recess may make an axial gap at the coupled area of cylindrical bearing and the nut member. As the result, feeding errors of the step actuator may become serious when the lead screw moves back and forth along the axial direction.
In an effort to solve the problems, a structure of coupling the nut member to the bearing through a swaging process after heating an end of the nut member or performing ultrasonic treatment on the end of the nut member was proposed by Korean Patent Application No. 10-2010-89709. Although this reduces the axial gap of the coupling area of the cylindrical bearing and the nut member to some extent, feeding errors still remain as the gap is not fully removed.